The significance of a fast and precise control of valve systems is increasing with an increase demand for hydraulic systems. One example of such a field of activity is fuel injection, for example direct injection of diesel fuel into the combustion chamber of a motor. What is referred to as the "common rail" system wherein the fuel is conveyed from a central conveying pump into a filling conduit ("common rail") shared by all cylinders thereby has great potential. The dosing of the fuel ensues via a system for fuel injection that is individually allocated to each and every cylinder. The improvement of the motor operating behavior that can be achieved with the assistance of a common rail injection system thereby essentially results from an injection pressure of up to 2500 bar that can be regulated independently of the motor speed. Added thereto given this technology is the possibility of shaping the course of the injection, i.e. of generating a single or multiple pilot injection, or of the control of the injection rate as well as the free control of characteristics of start of injection and injected amount.
For realizing these advantages, the system for fuel injection must satisfy a very high dynamic demand; for example, it must exhibit a short drive dead time and a short switching time.
Up to now, the control of common rail injectors has essentially ensued with the assistance of a solenoid drive. In some instances, the injector is also controlled with the assistance of a piezo-hydraulic drive.
In the control of a fuel injector with the assistance of a piezoelectric direct drive for valve control of the hydraulic system, the problem arises, for example, that only an inadequate compensation of a change in length of piezo actuator and housing caused by temperature effects or by aging and settling effects is realized. Added thereto is that a piezo actuator having a large structural length is required given piezo direct drive, which is disadvantageous in terms of manufacturing technology and in view of the manufacturing costs.
Numerous problems such as, for example, an involved mechanical balancing, a risk of breaking the diaphragm and well as a low efficiency of the diaphragm-type hydraulics arise given a combination of the piezo actuator with a diaphragm-type hydraulics for valve control in the injection system. Also unsatisfactory are, for example, the influence of pressure waves, a problematical temperature compensation as well as a merely satisfactory switching behavior.
Another example for the employment of a fast valve control is the braking circulation of a vehicle, whereby the hydraulic pressure in an anti-blocking system must be regulated precisely and fast. The employment of a fast and precise valve control in the hydraulic circulation of an elevator control or, respectively, vertical rudder in an aircraft is also conceivable. The guidance rudder must thereby be driven very fast in order to assure the safety of the aircraft, particularly in modern aircraft designed aerodynamically unstable.